ACCESSION #: 9408110043
LICENSEE EVENT REPORT (LER)
FACILITY NAME: Diablo Canyon, Unit 1 PAGE: 1 OF 6
DOCKET NUMBER: 05000275
TITLE: Failure to Implement Residual Heat Removal System
Overpressure Protection Requirements Due to Insufficient
Design Basis Information
EVENT DATE: 6/29/94 LER #: 94-015-00 REPORT DATE: 7/29/94
OTHER FACILITIES INVOLVED: Diablo Canyon, Unit 2 DOCKET NO: 05000323
OPERATING MODE: 1 POWER LEVEL: 100
THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR
SECTION:
50.73(a)(2)(v)(B)
LICENSEE CONTACT FOR THIS LER:
NAME: David P. Sisk - Senior Regulatory TELEPHONE: (805) 545-4420
Compliance Engineer
COMPONENT FAILURE DESCRIPTION:
CAUSE: SYSTEM: COMPONENT: MANUFACTURER:
REPORTABLE NPRDS:
SUPPLEMENTAL REPORT EXPECTED: NO
ABSTRACT:
On June 29, 1994, with Units 1 and 2 in Mode 1 (Power Operation), PG&E
determined that operation of the residual heat removal (RHR) system with
flow less than 3000 gpm in Mode 4 (Hot Shutdown) and 5 (Cold Shutdown -
with loops filled) may not meet ASME Code design basis overpressure
protection requirements during postulated pressure transients. At 1236
PDT, a four-hour, non-emergency report was made to the NRC in accordance
with 10 CFR 50.72(b)(2)(iii)(B).
During an RHR system evaluation, PG&E engineering personnel determined
that a postulated pressure transient could exceed the design pressure
limit required by ASME Section VIII, 1968, UG-125, "Pressure-Relief
Devices," paragraph (c).
The root cause of this condition was insufficient design basis
information provided in design basis documentation.
An engineering review of the design and operation of the RHR system will
be performed to ensure overpressure protection requirements are met. RHR
system design criteria memorandum will be revised to include
overpressurization design requirements. Procedures will be reviewed and
revised as necessary to ensure the RHR overpressurization design
requirements are satisfied. PG&E will request the NSSS vendor to clarify
the overpressure design basis requirements and provide a TS Bases
revision for the RHR system.
END OF ABSTRACT
TEXT PAGE 2 OF 6
I. Plant Conditions
Units 1 and 2 have operated in Modes 4 (Hot Shutdown) and 5 (Cold
Shutdown - with loops filled) with the potential for the condition
described below.
II. Description of Problem
A. Summary
On June 29, 1994, PG&E determined that operation of the
residual heat removal system (RHR) (BP) with a flow less than
3000 gpm in Mode 4 (Hot Shutdown) and 5 (Cold Shutdown - with
loops filled) may not meet ASME Code design basis overpressure
protection limits during postulated pressure transients. At
1236 PDT, a four-hour non-emergency report was made to the NRC
in accordance with 10 CFR 50.72(b)(2)(iii)(B).
B. Background
The RHR heat exchanger is constructed to ASME Section VIII,
1968, and described in the Final Safety Analysis Report (FSAR)
Update Section 5.5.6, "Residual Heat Removal System." The
relief valve located at the RHR pump suction line is provided
to comply with ASME Code requirements as described in FSAR
Update Section 5.5.6.3.3, "Overpressure Protection," and is
consistent with the requirements of Standard Review Plan
Section 5.4.7, "Residual Heat Removal (RHR) System," criteria,
detailed in "Branch Technical Position RSB 5-1, Design
Requirements of the Residual Heat Removal System."
RSB 5-1, Branch Position C., "Pressure Relief Requirements,"
indicates the RHR system shall satisfy the following: to
protect the RHR system against accidental overpressurization
when it is in operation (not isolated from the RCS), pressure
relief in the RHR system shall be provided with relieving
capacity in accordance with the ASME Boiler and Pressure Vessel
Code. The most limiting pressure transient during the plant
operating condition when the RHR system is not isolated from
the RCS shall be considered when selecting the pressure
relieving capacity of the RHR system. For example, during
shutdown cooling in a PWR with no steam bubble in the
pressurizer, inadvertent operation of an additional charging
pump or inadvertent opening of an ECCS accumulator valve should
be considered in the selection of the design bases.
The RHR pressure relief requirements are specified in ASME
Section VIII, 1968, UG-125, "Pressure-Relief Devices,"
paragraph (c), which indicates all pressure vessels shall be
protected by pressure relieving devices that will prevent the
pressure from rising more than 10 percent above the maximum
allowable working pressure.
The working pressure of the RHR system is 600 psig and the
suction relief valve setpoint is established at approximately
450 psig. In accordance with Code requirements, the relief
valve setpoint plus 10 percent accumulation pressure must be
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considered for protection of downstream components, i.e.,
piping and heat exchanger. Therefore, Code pressure relief
requirements must consider the sum of 495 psig at the suction
relief valve, plus the operating RHR pump pressure increase,
minus piping line losses, and not exceed a total of 660 psig.
C. Event Description
On May 1, 1994, with Unit 1 in Mode 5, reactor coolant system
(RCS) at 350 psig, 190 degrees Fahrenheit and the pressurizer
water solid, the pressure rapidly increased due to a charging
flow increase to the RCS during the performance of a routine
charging system vent. The event resulted in the rapid increase
in RCS and RHR pressure. Plant operators responded to alarms
in the control room and isolated the source of the additional
charging and the pressure transient was terminated below the
actuation setpoint (RCS pressure of 450 psig) of the low
temperature overpressure protection (LTOP) system. However, an
RHR system pressure high alarm with a setpoint of 600 psig was
received during the transient (a maximum of 605 psig in the RHR
system). Although the actual pressure transient was terminated
below the LTOP actuation pressure, an evaluation was initiated
to further investigate implications of RHR pressure transients.
On June 29, 1994, additional engineering calculations performed
for the RHR system during pressure transient conditions
determined that at flows less than 3000 gpm, design basis
overpressure protection limits at the RHR heat exchanger may
not be met.
On June 29, 1994, at 1236 PDT, a four-hour non-emergency report
was made in accordance with 10 CFR 50.73(b)(2)(iii)(B).
D. Inoperable Structures, Components, or Systems that Contributed
to the Event
None.
E. Dates and Approximate Times for Major Occurrences
1. May 1, 1994: A momentary RCS pressure transient
occurred.
2. June 29, 1994: Event/Discovery Date: Engineering
calculations conclude that ASME Code
relief protection requirements may not
be met during RHR low flow operation.
3. June 29, 1994, at A four-hour non-emergency report
was 1236 PDT: made to the NRC in accordance with
10 CFR 50.72(b)(2)(iii)(B).
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TEXT PAGE 4 OF 6
F. Other Systems or Secondary Functions Affected
None
G. Method of Discovery
As a result of investigations performed regarding a May 1,
1994, RCS pressure transient, PG&E personnel performed detailed
pressure calculations for the RHR system which determined that
design basis pressures could be exceeded for postulated
pressure transients in Modes 4 and 5.
H. Operator Actions
None.
I. Safety System Responses
None.
III. Cause of the Problem
A. Immediate Cause
The immediate cause of this problem was inadequate control of
RHR flow to assure the ASME Code overpressure protection for
all operating conditions.
B. Root Cause
The root cause of this condition was insufficient design basis
information for the RHR system overpressure protection relief
requirements during postulated transient conditions in Modes 4
and 5 (with loops filled). The level of detail provided in the
NSSS vendor design basis documents was inadequate regarding
ASME Code overpressure requirements. As a result, procedural
guidance was not established to ensure RHR overpressure
protection requirements were met.
IV. Analysis of the Event
The most limiting overpressure transient is postulated to occur
during shutdown conditions with no steam bubble in the pressurizer
(water solid conditions) when inadvertent operation of additional
charging injection could cause a rapid rise in RCS pressure for
which Code pressure relief requirements are applicable and RHR flow
was maintained at 1300 gpm (minimum flow per TS 4.9.8.1.2).
PG&E conservatively evaluated a limiting pressure transient with
1200 gpm RHR flow and determined that the maximum pressure at the
RHR heat exchanger would be approximately 685 psig. This pressure
is above of the 660 psig maximum design limit and does not meet Code
requirements. The maximum postulated pressure is within the 125
percent of the
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TEXT PAGE 5 OF 6
inservice test pressure, the 150 percent hydro test pressure, and
well within the strength of materials such that postulated damage
would not occur.
Therefore, the health and safety of the public were not adversely
affected by this event.
V. Corrective Actions
A. Immediate Corrective Actions
1. An operation incident summary was issued to all plant
operators regarding the May 1, 1994, event to inform
operating personnel of the pressure transient and the need
to prevent operational challenges to systems.
2. An operations shift order was issued to all plant
operators regarding the engineering evaluation of the RHR
system and need to assure adequate flow to protect the
heat exchanger from exceeding its maximum design pressure
in Modes 4 and 5 (with RCS loops filled).
B. Corrective Actions to Prevent Recurrence
1. PG&E engineering will revise the RHR design criteria
memorandum (DCM) to include requirements regarding
overpressure protection of the system during Modes 4 and 5
(with RCS loops filled).
2. PG&E operations will review and revise plant operating
procedures as necessary to assure RHR design basis
overpressure protection requirements are satisfied.
3. A TS 3/4.4.1 bases revision will be made to provide the
design and operational basis regarding ASME Code
overpressure protection requirements.
4. PG&E engineering will review design bases and operating
configurations to ensure RHR overpressure protection
requirements are met.
5. PG&E will formally request Westinghouse to provide
information to utilities regarding the effect of RHR flow
on overpressure protection requirements.
VI. Additional Information
A. Failed Components
None.
B. Previous LERs on Similar Problems
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TEXT PAGE 6 OF 6
LER 1-93-008 (Voluntary), "Block Valves Installed on
Inlet/Discharge Side of Overpressure Protection Devices Due to
Vendor Design Deficiency," identified a condition where manual
block valves had been provided in the pressure protection flow
stream contrary to Code requirements. The corrective actions
for this LER included transmission of information to the
original supplier and submission of a relief request for the
specific equipment.
As a result of LER 1-93-008 and other considerations, PG&E
management initiated additional Code training for system
engineers and reviewers involved with Code issues as a process
enhancement. This ongoing training will add to the heightened
awareness of PG&E personnel to a broad range of Code
requirements.
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ATTACHMENT TO 9408110043 PAGE 1 OF 1
Pacific Gas and Electric Company
77 Beale Street, Room 1451 Gregory M. Rueger
P.O. Box 770000 Senior Vice President and
San Francisco, CA 94177 General Manager
415/973-4684 Nuclear Power Generation
Fax 415/973-2313
July 29, 1994
PG&E Letter DCL-94-163
U.S. Nuclear Regulatory Commission
ATTN: Document Control Desk
Washington, D.C. 20555
Docket No. 50-275, OL-DPR-80
Docket No. 50-323, OL-DPR-82
Diablo Canyon Units 1 and 2
Licensee Event Report 1-94-015-00
Failure to Implement Residual Heat Removal System Overpressure Protection
Requirements Due to Insufficient Design Basis Information
Gentlemen:
Pursuant to 10 CFR 50.73(a)(2)(v)(B), PG&E is submitting the enclosed
Licensee Event Report regarding failure to implement residual heat
removal system overpressure protection requirements due to insufficient
design basis information.
This event did not adversely affect the health and safety of the public.
Sincerely,
Gregory M. Rueger
cc: L. J. Callan
Mary H. Miller
Kenneth E. Perkins
Sheri R. Peterson
Diablo Distribution
INPO
Enclosure
DC0-94-EN-N016
1216S/DDM/2246
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